1. Field of the Invention:
This invention relates to methods of and apparatus for transmitting signals over the lines of an alternating current power distribution network using audio-frequency signals.
2. Description of the Prior Art:
The best known method of transmitting signals over the lines of an alternating current power supply network is the audio-frequency power line carrier control method, otherwise known as the ripple control method. In this method, audio-frequency signals of small band-width are transmitted from one or a few centrally located transmitters over the power supply network, and are evaluated in a plurality of receivers distributed about the network, for carrying out instructions of different types. In this method, the transmission speed and consequently the amount of information transmitted per unit of time is relatively small.
Methods are also known which, in many respects, are similar to that described immediately above, and in which for example the counter state of counters distributed about the network, or information concerning the carrying out of instructions in the audio-frequency power line carrier control method are reported back to a central station. Throughout this specification instructions and information will be referred to collectively as "intelligence". Reporting takes place in the opposite direction to the flow of power, and for reporting back it is necessary to have a plurality of transmitters with a relatively small capability, and which must be cheap, whilst there is only one or a few centrally located receivers, which may therefore be relatively expensive.
It has therefore been proposed to evaluate the indication of consumption of fixed quantitites of consumed energy, gas, water and the like with the aid of instructions released by audio-frequency power line carrier control signals for the instantaneous coupling of a resonant circuit between two network conductors in the individual counters and by evaluating the signals produced in a station receiver having extremely selective electronic filters (Austrian Pat. No. 241 589). A transmitter of the type indicated hereinbefore suitable for this purpose has also been proposed (Swiss Pat. No. 446 962). This transmitter generates resonant oscillations in a resonant circuit tuned to the audio-frequency, which oscillations decay after a few cycles. Thus, the amplitude of the oscillations in this resonant circuit and in the network supplied therewith is not constant, so that the use of such a transmitter is limited due to the inadequately defined remote control frequencies produced as a result of the unpredictable changing states of the network and the lack of possibility for modulation.